diff --git includes/bootstrap.inc includes/bootstrap.inc
index 9060a0e..1e446f0 100644
--- includes/bootstrap.inc
+++ includes/bootstrap.inc
@@ -2244,7 +2244,6 @@ function _drupal_bootstrap_database() {
   // The database autoload routine comes first so that we can load the database
   // system without hitting the database. That is especially important during
   // the install or upgrade process.
-  spl_autoload_register('db_autoload');
   spl_autoload_register('drupal_autoload_class');
   spl_autoload_register('drupal_autoload_interface');
 }
diff --git includes/database/database.inc includes/database/database.inc
index 98dafae..c85e35a 100644
--- includes/database/database.inc
+++ includes/database/database.inc
@@ -648,13 +648,20 @@ abstract class DatabaseConnection extends PDO {
    *
    * @param string $class
    *   The class for which we want the potentially driver-specific class.
+   * @param array $files
+   *   The name of the files in which the driver-specific class can be.
+   * @param $use_autoload
+   *   If TRUE, attempt to load classes using PHP's autoload capability
+   *   as well as the manual approach here.
    * @return string
    *   The name of the class that should be used for this driver.
    */
-  public function getDriverClass($class) {
+  public function getDriverClass($class, array $files = array(), $use_autoload = FALSE) {
     if (empty($this->driverClasses[$class])) {
-      $this->driverClasses[$class] = $class . '_' . $this->driver();
-      if (!class_exists($this->driverClasses[$class])) {
+      $driver = $this->driver();
+      $this->driverClasses[$class] = $class . '_' . $driver;
+      Database::loadDriverFile($driver, $files);
+      if (!class_exists($this->driverClasses[$class], $use_autoload)) {
         $this->driverClasses[$class] = $class;
       }
     }
@@ -681,7 +688,7 @@ abstract class DatabaseConnection extends PDO {
    * @see SelectQuery
    */
   public function select($table, $alias = NULL, array $options = array()) {
-    $class = $this->getDriverClass('SelectQuery');
+    $class = $this->getDriverClass('SelectQuery', array('query.inc', 'select.inc'));
     return new $class($table, $alias, $this, $options);
   }
 
@@ -697,7 +704,7 @@ abstract class DatabaseConnection extends PDO {
    * @see InsertQuery
    */
   public function insert($table, array $options = array()) {
-    $class = $this->getDriverClass('InsertQuery');
+    $class = $this->getDriverClass('InsertQuery', array('query.inc'));
     return new $class($this, $table, $options);
   }
 
@@ -713,7 +720,7 @@ abstract class DatabaseConnection extends PDO {
    * @see MergeQuery
    */
   public function merge($table, array $options = array()) {
-    $class = $this->getDriverClass('MergeQuery');
+    $class = $this->getDriverClass('MergeQuery', array('query.inc'));
     return new $class($this, $table, $options);
   }
 
@@ -730,7 +737,7 @@ abstract class DatabaseConnection extends PDO {
    * @see UpdateQuery
    */
   public function update($table, array $options = array()) {
-    $class = $this->getDriverClass('UpdateQuery');
+    $class = $this->getDriverClass('UpdateQuery', array('query.inc'));
     return new $class($this, $table, $options);
   }
 
@@ -746,7 +753,7 @@ abstract class DatabaseConnection extends PDO {
    * @see DeleteQuery
    */
   public function delete($table, array $options = array()) {
-    $class = $this->getDriverClass('DeleteQuery');
+    $class = $this->getDriverClass('DeleteQuery', array('query.inc'));
     return new $class($this, $table, $options);
   }
 
@@ -762,7 +769,7 @@ abstract class DatabaseConnection extends PDO {
    * @see TruncateQuery
    */
   public function truncate($table, array $options = array()) {
-    $class = $this->getDriverClass('TruncateQuery');
+    $class = $this->getDriverClass('TruncateQuery', array('query.inc'));
     return new $class($this, $table, $options);
   }
 
@@ -776,7 +783,7 @@ abstract class DatabaseConnection extends PDO {
    */
   public function schema() {
     if (empty($this->schema)) {
-      $class = $this->getDriverClass('DatabaseSchema');
+      $class = $this->getDriverClass('DatabaseSchema', array('schema.inc'));
       if (class_exists($class)) {
         $this->schema = new $class($this);
       }
@@ -1581,6 +1588,34 @@ abstract class Database {
     self::$ignoreTargets[$key][$target] = TRUE;
   }
 
+  /**
+   * Load a file for the database that might hold a class.
+   *
+   * @param $driver
+   *   The name of the driver.
+   * @param array $files
+   *   The name of the files the driver specific class can be.
+   */
+  public static function loadDriverFile($driver, array $files = array()) {
+    static $base_path;
+
+    if (empty($base_path)) {
+      $base_path = dirname(realpath(__FILE__));
+    }
+
+    $driver_base_path = "$base_path/$driver";
+    foreach ($files as $file) {
+      // Load the base file first so that classes extending base classes will
+      // have the base class loaded.
+      foreach (array("$base_path/$file", "$driver_base_path/$file") as $filename) {
+        // The OS caches file_exists() and PHP caches require_once(), so
+        // we'll let both of those take care of performance here.
+        if (file_exists($filename)) {
+          require_once $filename;
+        }
+      }
+    }
+  }
 }
 
 /**
@@ -2109,82 +2144,6 @@ class DatabaseStatementEmpty implements Iterator, DatabaseStatementInterface {
 }
 
 /**
- * Autoload callback for the database system.
- */
-function db_autoload($class) {
-  static $base_path = '';
-  static $checked = array();
-
-  static $files = array(
-    'query.inc' => array(
-      'QueryPlaceholderInterface',
-      'QueryConditionInterface', 'DatabaseCondition',
-      'Query', 'DeleteQuery', 'InsertQuery', 'UpdateQuery', 'MergeQuery', 'TruncateQuery',
-      'QueryAlterableInterface',
-    ),
-    'select.inc' => array('QueryAlterableInterface', 'SelectQueryInterface', 'SelectQuery', 'SelectQueryExtender'),
-    'database.inc' => array('DatabaseConnection'),
-    'log.inc' => array('DatabaseLog'),
-    'prefetch.inc' => array('DatabaseStatementPrefetch'),
-    'schema.inc' => array('DatabaseSchema'),
-  );
-
-  // If a class doesn't exist, it may get checked a second time
-  // by class_exists().  If so, just bail out now.
-  if (isset($checked[$class])) {
-    return;
-  }
-  $checked[$class] = TRUE;
-
-  if (empty($base_path)) {
-    $base_path = dirname(realpath(__FILE__));
-  }
-
-  // If there is an underscore in the class name, we know it's a
-  // driver-specific file so check for those.  If not, it's a generic.
-  // Note that we use require_once here instead of require because of a
-  // quirk in class_exists().  By default, class_exists() will try to
-  // autoload a class if it's not found.  However, we cannot tell
-  // at this point whether or not the class is going to exist, only
-  // the file that it would be in if it does exist.  That means we may
-  // try to include a file that was already included by another
-  // autoload call, which would break.  Using require_once() neatly
-  // avoids that issue.
-  if (strpos($class, '_') !== FALSE) {
-    list($base, $driver) = explode('_', $class);
-
-    // Drivers have an extra file, and may put their SelectQuery implementation
-    // in the main query file since it's so small.
-    $driver_files = $files;
-    $driver_files['query.inc'][] = 'SelectQuery';
-    $driver_files['install.inc'] = array('DatabaseTasks');
-
-    foreach ($driver_files as $file => $classes) {
-      if (in_array($base, $classes)) {
-        $filename = "{$base_path}/{$driver}/{$file}";
-        // We might end up looking in a file that doesn't exist, so check that.
-        if (file_exists($filename)) {
-          require_once $filename;
-          // If the class now exists, we're done. Otherwise keep searching in
-          // additional files.
-          if (class_exists($class, FALSE) || interface_exists($class, FALSE)) {
-            return;
-          }
-        }
-      }
-    }
-  }
-  else {
-    foreach ($files as $file => $classes) {
-      if (in_array($class, $classes)) {
-        require_once $base_path . '/' . $file;
-        return;
-      }
-    }
-  }
-}
-
-/**
  * The following utility functions are simply convenience wrappers.
  *
  * They should never, ever have any database-specific code in them.
@@ -2915,4 +2874,3 @@ function db_ignore_slave() {
     $_SESSION['ignore_slave_server'] = REQUEST_TIME + $duration;
   }
 }
-
diff --git includes/database/schema.inc includes/database/schema.inc
index 187f9e6..36c7964 100644
--- includes/database/schema.inc
+++ includes/database/schema.inc
@@ -6,6 +6,8 @@
  * Generic Database schema code.
  */
 
+require_once dirname(__FILE__) . '/query.inc';
+
 /**
  * @defgroup schemaapi Schema API
  * @{
diff --git includes/database/select.inc includes/database/select.inc
index 7780fc5..b0d0eb2 100644
--- includes/database/select.inc
+++ includes/database/select.inc
@@ -6,6 +6,8 @@
  * @{
  */
 
+require_once dirname(__FILE__) . '/query.inc';
+
 /**
  * Interface for extendable query objects.
  *
@@ -642,7 +644,9 @@ class SelectQueryExtender implements SelectQueryInterface {
   /* Implementations of QueryExtendableInterface. */
 
   public function extend($extender_name) {
-    $class = $this->connection->getDriverClass($extender_name);
+    // The extender can be anywhere so this needs to go to the registry, which
+    // is surely loaded by now.
+    $class = $this->connection->getDriverClass($extender_name, array(), TRUE);
     return new $class($this, $this->connection);
   }
 
diff --git includes/install.core.inc includes/install.core.inc
index 179d6b8..2c734c5 100644
--- includes/install.core.inc
+++ includes/install.core.inc
@@ -289,7 +289,6 @@ function install_begin_request(&$install_state) {
     // Initialize the database system. Note that the connection
     // won't be initialized until it is actually requested.
     require_once DRUPAL_ROOT . '/includes/database/database.inc';
-    spl_autoload_register('db_autoload');
 
     // Verify the last completed task in the database, if there is one.
     $task = install_verify_completed_task();
diff --git includes/install.inc includes/install.inc
index 6e0b7dc..977a654 100644
--- includes/install.inc
+++ includes/install.inc
@@ -238,7 +238,6 @@ function drupal_detect_database_types() {
   // Because we have no registry yet, we need to also include the install.inc
   // file for the driver explicitly.
   require_once DRUPAL_ROOT . '/includes/database/database.inc';
-  spl_autoload_register('db_autoload');
   foreach (file_scan_directory(DRUPAL_ROOT . '/includes/database', '/^[a-z]*$/i', array('recurse' => FALSE)) as $file) {
     if (file_exists($file->uri . '/database.inc') && file_exists($file->uri . '/install.inc')) {
       $drivers[$file->filename] = $file->uri;
@@ -246,8 +245,7 @@ function drupal_detect_database_types() {
   }
 
   foreach ($drivers as $driver => $file) {
-    $class = 'DatabaseTasks_' . $driver;
-    $installer = new $class();
+    $installer = db_installer_object($driver);
     if ($installer->installable()) {
       $databases[$driver] = $installer->name();
     }
@@ -1123,8 +1121,18 @@ function install_profile_info($profile, $locale = 'en') {
  * encoding.
  */
 function db_run_tasks($driver) {
-  $task_class = 'DatabaseTasks_' . $driver;
-  $DatabaseTasks = new $task_class();
-  $DatabaseTasks->runTasks();
+  db_installer_object($driver)->runTasks();
   return TRUE;
 }
+
+/**
+ * Returns a database installer object.
+ *
+ * @param $driver
+ *   The name of the driver.
+ */
+function db_installer_object($driver) {
+  Database::loadDriverFile($driver, array('install.inc'));
+  $task_class = 'DatabaseTasks_' . $driver;
+  return new $task_class();
+}
diff --git includes/theme.maintenance.inc includes/theme.maintenance.inc
index 7cc0312..4950d82 100644
--- includes/theme.maintenance.inc
+++ includes/theme.maintenance.inc
@@ -41,7 +41,6 @@ function _drupal_maintenance_theme() {
     // to work. See _drupal_log_error().
     if (!class_exists('Database', FALSE)) {
       require_once DRUPAL_ROOT . '/includes/database/database.inc';
-      spl_autoload_register('db_autoload');
     }
 
     // We use the default theme as the maintenance theme. If a default theme
diff --git modules/simpletest/drupal_web_test_case.php modules/simpletest/drupal_web_test_case.php
index 1949b99..39cd7e0 100644
--- modules/simpletest/drupal_web_test_case.php
+++ modules/simpletest/drupal_web_test_case.php
@@ -597,8 +597,6 @@ class DrupalUnitTestCase extends DrupalTestCase {
     // Store necessary current values before switching to the test environment.
     $this->originalFileDirectory = variable_get('file_public_path', conf_path() . '/files');
 
-    spl_autoload_register('db_autoload');
-
     // Reset all statics so that test is performed with a clean environment.
     drupal_static_reset();